This guide provides step-by-step support for organising a Climathon, a participatory event format that brings together students, researchers, educators and local stakeholders to explore climate challenges and co-develop solutions. Developed within ESM2025, it outlines key stages of planning and delivery, including defining themes, engaging participants, structuring activities, facilitating discussion, and showcasing outcomes. The guide emphasises the use of up-to-date climate science as a foundation for dialogue, while encouraging interdisciplinary exchange and local relevance. Designed for use in education, outreach and stakeholder engagement contexts, it supports organisers in creating inclusive, interactive events that strengthen climate literacy, foster collaboration and link scientific knowledge with real-world climate action.